LD50 is defined as the lethal dose for 50% of a test population.

• A. The lethal dose for 50% of the test population
• B. The lethal dose for 100% of the test population
• C. The lethal dose for 25% of the test population
• D. The lethal dose for 0% of the test population

Answer: (A)

LD50 is the dose that kills 50% of a defined test population under specific conditions. It’s a median value drawn from dose–response data, used to compare how acutely toxic different substances are. This isn’t the dose that kills everyone or none of them; it represents the point at which half of those exposed would be expected to die. The exact LD50 depends on factors like species, route of exposure, and experimental conditions, so different tests can yield different numbers for the same chemical. A lower LD50 means higher acute toxicity because only a small amount is needed to reach that 50% mortality threshold. For example, a chemical with an LD50 of 50 mg/kg in rats by oral exposure is more toxic than one with an LD50 of 500 mg/kg.